In a semiconductor laser array in which a plurality of semiconductor laser elements as one of semiconductor light emitting elements are arranged, a laser output is generally saturated due to a temperature increase. Therefore, a heat sink which cools the semiconductor laser array is used with the semiconductor laser array. For instance, these techniques are disclosed in “O plus E 1999-2 Vol. 21 No. 2(p. 173-178)” and Japanese Patent Application Laid-Open No. H8-139479. According to the techniques, the semiconductor laser array is cooled by cooling water passing through a cooling water passage provided in the heat sink, and thereby the laser output is prevented from being saturated.